1. Field of the Invention
The present invention relates to a bias circuit of a high frequency electronic circuit having enhancement FET formed on a semiconductor substrate, such as GaAs, and in particular to a bias circuit for supplying a bias voltage associated with a transistor characteristic, even though the transistor characteristic varies due to differences during the manufacturing process.
2. Related Arts
An MMIC (Microwave Monolithic Integrated Circuit) wherefor a plurality of metal gate FET devices are integrated on a semiconductor substrate, such as GaAs or Si, is commonly utilized to provide a compact hardware for processing a high frequency signal in, for example, a portable telephone. A MMIC circuit is employed, for example, as an amplifier to amplify power relative to high frequency input and generate a high powered high frequency output, or as a mixer circuit that converts a high frequency signal it received to a low frequency signal.
FIG. 10 is a diagram showing a common high frequency amplifier and its bias circuit. An electronic circuit 10 includes a high frequency amplifier, and a bias circuit 20 generates bias voltages V1 and V2 having a constant potential that are supplied to the electronic circuit 10. In the electronic circuit 10 in this example, a load R1 and an amplification transistor X1 are connected between a positive power source V.sub.DD and the ground. A high frequency input RFin is supplied to the gate of the transistor X1 and a high frequency output RFout is generated at its drain. In addition, bias voltage V1 is applied to the gate. With this arrangement, a signal which the high frequency input RFin is superimposed on the bias voltage V1 is applied between the gate and the source, and the high frequency output RFout, obtained by amplification, is generated at the drain terminal.
In the bias circuit in this example, resistors R41, R42, R43 and R44 are connected in series between the positive voltage source V.sub.DD and the ground, and in this example, a constant voltage divided by the resistances is generated at nodes N41 and N42. The voltages at nodes N41 and N42 are supplied via the respective resistors R45 and R46 to the electronic circuit 10.
In the bias circuit 20 of a resistance division type shown in FIG. 10, the bias voltages V1 and V2 are almost constant. However, the MMIC generates, for example, a metal gate electrode relative to an active layer having a predetermined thickness on the surface of the substrate, and applies a voltage between the gate and the source, so that an depletion extending from the gate electrode to the active layer is expanded or narrowed to control a drain current value. Since during the manufacturing process the thicknesses of the active layer and the metal gate electrode vary in manufacture, the characteristic of the bias circuit 20 tends to fluctuate.
For the high frequency amplification transistor X1 in FIG. 10, therefore, the static characteristics, such as pinch-off voltage Vp and drain saturated current I.sub.DSS, are greatly changed due to the manufacturing differences. And thus, when the bias voltage V1 to be applied to the gate is constant, as is described above, an appropriate bias voltage can not be applied relative to the varying static characteristic of the FET.times.1, an amplification rate and a dynamic range are changed, a drain current of a design value can not be obtained, and a desired characteristic for the operation of a high frequency amplifier can not be provided.